Wide frequency band amplifier system



Dec. 16, 1941. w, HANSELL 2,266,197

WIDE FREQiIENCY BAND AMPLIFIER SYSTEM Filed Nov. 26, 1958 TRANSMISSIONLIA/E r0 ANTENNA I III WAVE 0/? 000 Ml/LT/PLE THEREOF HIGH 3 FREQUENCYL. 04mm? mlpur R Q I g TWO 5120,40 3 BAND AMPLIFIERS 2' Q 80 s 70 0/Vsaw/m0 AMPL/F/ER 2 60 R50 E IE40 w l l l I I I I MEG/ICYCL E5 I r0ANTENNA Eg 3 AMPl/F/ER 5721658 I l "I I'TJ 'I I I I I LINE I F I I ,1 ll s n/0M9 B I I I I I A 3 LINE 6 l I I 4 l I 556770; l I I A 4 LINE 5 Il l I I I sin/0M5 I I I L I I l I J INVENTOR.

L I "CZAZ/CEWHANSELI.

I BY (ARR/ER FREQUENCY INPUT wv-w A TTORNEY.

Patented Dec. 16, 1941 UNITED STATES PATENT, 'oF lc V WIDE mmurgggg snAMPLIFIER I tion of Delaware Application November 26, 1938, Serial No.242,442

17 Claims.

The present invention relates to electron discharge device amplifiers ofhigh frequency currents, and particularly to vacuum tube amplifiers foramplifying a wide band of frequencies.

The invention is particularly applicable to radio transmitters andreceivers, especially for the transmission and reception of televisionsignals, and will be described with regard thereto, although it is to bedistinctly understood that the invention is not limited in this respect,but may be used wherever there is need for a wide frequency bandamplifier.

In some kinds of radio transmitters, particularly those used fortelevision transmission, it is extremely difficult to obtain suflicientbroad band frequency response in the modulated high frequency circuits.A similarly diflicult problem has existed in the receivers.

The lack of sufiicient band width in the high.

frequency amplifier circuits has been a result of the dielectriccapacity of electron tubes and circuits. This capacity is balanced out,by tuning with inductance, for some frequency in the transmitted band,which is usually the carrier frequency. For any other frequency, aboveor below the resonant frequency, the capacity and inductance no longerhave equal reactance and do not balance out. At resonance the amplifiervacuum tubes deliver power to an output circuit which is substantiallyan effective resistance but, for higher or lower frequencies the outputcircuit becomes reactive, as well as resistive, and the vacuum tubesmust deliver output current to supply a reactive component as well as aresistive component. This requires an increase in electron currentthrough the tube to supply the reactive component of output current. atfrequencies different from the resonant. frequency. In practice it hasbeen, so far, impossible to build vacuum tubes with enough usefulelectron emission and peak electron current to supply the unbalancedcomponent of reactive current to the output circuit, in addition to theresistive component, over a frequency band wide enoughfor reallysatisfactory television. Amplifier tubes are needed with a much greaterratio'of peak available electron current to interelectrode dielectriccapacity,

but inspite of the great effort and huge expendi-.,

tures for development of better tubes, only very slow progress has beenmade and the best of the existing tubes, although they are not goodenough, seem to. have been pushed very close to the ultimate limits forthis kind of device.

The present. invention recognizesthis fundaelectron current todielectric capacity between electrodes and provides a'means for wideningthe frequency band of amplifiers without further improvement in thevacuum tubes. Briefly, the manner of bringing aboutthislimprovement inband width and frequency response is to employ two amplifiers whichcooperate not only to deliver a useful output of modulated highfrequency energybut whichalso cooperate-to reduce the reactive componentof currentrequired from each amplifier. To obtain this result the outputcir. cuits of the two amplifiers of-the invention are joined togetherthrough a section of real or artilficial transmission line ofsuitable-characteristic impedance, which is electrically 'a quarter waveat the center of the frequency bandtobe transmitted. or in some cases anodd number of quarter waves long. Where double side band transmission isemployed-thelength'of the line will be mental limit in the obtainableratio of us ful peak 55 electrically and. effectively a quarter wave,or'an oddmultiple of a quarter wave, at the .carrier frequency.' The twoamplifiers are thenprovided with high frequency inputs which are 90different in phase. Theoutput from the two amplifiers is taken from thatend of the quarter wave ment as possible-the invention provides an im--pedance inverter in the. form of. a quarter-wavelength line, or oddmultiple thereof,.placed be tween a pair of amplifiers toachievean-elfective bigoadening, of the frequency response character- 1s1c.

A better understanding'of the invention may be had by referringto thefollowing description, which is accompanied bye, drawing, wherein: I

Fig... 1 shows, by way of exampleonly, the prin ciples of the inventionapplied to apair of amplifiers in the last stage of a televisiontransmiter; Fig. 2 shows, graphically, the improvement obtained in usinga pair of amplifiers in accordance with the invention over a singleamplifier, in one embodiment successfully used in practice; and

Fig. 3 shows diagrammatically, in box form, how the system of Fig. 1 canbeused in a cascaded amplifier system.

Referring to Fig. 1 in more: detail, there are shown two amplifiers Aand B,each comprising a pair of push-pull connected, cross-neutralized,evacuated electron discharge devices I and 2. The input circuits 3 and4, as well as the output circuits 5 and 6 of the two amplifiers A and Bare, respectively, interconnected by quarter wavelength lines TL and TL,so as to provide a 90 phase relation therebetween.

Both input circuits 3 and 4 are supplied with an equal amount of highfrequency carrier current by means of transformers, and it is assumedthat the input circuit 3 of amplifier A is 90 leading in phase withrespect to input circuit 4 of amplifier B. This 90 phase relation isshown obtained by joining both input circuits with a quarter wavetransmission line TL which has its characteristic impedance matched atamplifier B. If desired, of course, the transformers can be omitted, andthe carrier frequency current applied directly to the grids of theamplifiers A and B, in association with suitable tuning and loadingmeans to obtain impedances equal to the characteristic impedance of theline section TL. The line TL is made to be a quarter wavelength long,electrically, at the mid frequency of the band to be amplified. Toobtain this condition, the physical length will usually be slightly lessthan a quarter wave. It will be understood, of course, that othermethods of obtaining the 90 phase relation" between input circuits 3 and4 can be used. For example, the two inputs 3 and 4 can-be coupled to acommon source of carrier frequency without a. quarter wave line betweenthem, andthe phase relations of their currents controlled by detuningthe two input circuits in opposite directions so as to obtain thedesired 90 phase relation. Also, when the line section is used, variabletuning may be used for obtaining more exact desired phase relations.

' The two tubes l and 2 of each amplifier are shown provided withneutralizing condensers coupled inknown manner between the anode of onetube and the grid of the associated tube. In orderto reduce the possibleeffects of imperfect amplifier neutralizing or balancing and also toreduce the effects of modulation upon the phase and amplitude of theexcitation power, the input circuits are preferably made to haveconsiderable resistive loading at the grids, as shown by resistances R.Also, when wide band modulation is to be applied to the anodes or gridsof the amplifier tubes, it is desirable to employ rather large capacityand low inductance for tuning the grids so as to produce considerablecirculating current in the input circuits. The grids of the tubes, itshould be noted, are maintained at a negative potential with respect totheir cathodes by means of grid bias lead l5 extending to the negativeterminal of a source of bias potential, not shown. The outputs from thetwo amplifiers are shown joined together through aquarter wavetransmission line TL which is preferably connected as nearly as possiblebetween the two vacuum tube anodes in each amplifier at each end of theline. Line TL is made to be a quarter wavelength long or an odd multipleof a quarter wavelength at the center of the band of frequencies to betransmitted. Another transmission line T'L' mission line T'L' extendingto the antenna. The output circuit of each amplifier A and B preferablyconsists of a parallel tuned circuit of inductance and capacitance,although a tuned lecher wire circuit can be employed instead.

In order to effect grid modulation of the amplifiers A and B, the lowfrequency grid potentials of the tubes may be controlled by suitablecircuits coupled to lead l5, and these grid potentials varied togetherby means of direct current and alternating current video frequencymodulation potentials so as to vary the outputs from connected to theantenna and having halfthe characteristic impedance of the first quarter7 wave line section is connected to the end of the quarter wave line' TLat the point of attachment to the anodes of the tubes of the amplifier Bwith the lagging phase. 5

The whole system is so designed and adjusted that the two amplifiers Aand B are tuned to resonance at a mid output frequency, which ispreferably the input carrier frequency, where double side bandtransmission is employed. The two. amplifiers A and B should deliverequal amounts of output power to the main transboth amplifiers inaccordance with the television signals to be transmitted. Instead ofgrid modulation,the high frequency carrier input may be modulated byintroducing modulation ahead of the last stage.

In many cases it may be preferable to introduce the modulation into theanode supply lead for, the amplifiers A and B. The choice of thelocation for introducing the modulation depends upon many factorsincluding difficulties encountered by the designer in providing highpower modulation equipment.

If now, assuming grid modulation, the grid bias on the amplifiers isvaried by video frequency input, the two amplifiers will have theiroutputs changed or modulated accordingly and both outputs will remainsubstantially equal for any valueof output, that is, both amplifiers arein parallel relation for the modulation although differing in phase byfor radio frequency output.

If the modulation produces a current component of frequency above orbelow the frequency to which each circuit is tuned, then each of theoutput circuits for the two ampliers will show a component of reactiveimpedance and the reactive component of current will be greater'andgreater the farther we go from the carrier frequency to which each ofthe circuits is tuned. In amplifiers of the prior art, this reactivecomponent of current flowing to eachof the amplifier output circuitswould have to be supplied by the tube, or the modulation responsecharacteristic would suffer. That is, the output at the off resonancefrequency would be reduced and shifted in phase with respect to thecarrier with corresponding changes in the modulation of the carrierrepresented by these off resonance frequencies. In'the system of theinvention, however, the amplifiers react upon one another through thequarter wave line section TL in such a way as to eliminate or greatlyreduce the reactive component of current required from the tubes.

If we assume that a high frequency component of current exists in theamplifier outputs which is at a higher frequency than the resonantfrequency, then each individual output circuit taken alone would presentan equivalent capacitive reactance and a resistance to the tubes. In thesystem of the invention, however, the capacitive reactance of eachamplifier circuit appears at the other amplifier circuit as anequivalent inductive reactance by virtue of the quarter wave line TLbetween them. Consequently, the reactive component of current requiredfrom the tubes is very greatly reduced because the reactive componentfor each circuit is balanced by reactive current exchanged through thequarter wave line TL.

The balancing of reactance in the two output circuits of the amplifiersAand B of the invention is not quite perfect and becomes less and lessperfect-as we depart far from resonance because the quarter wave line isno longer a 7 tion represents a very great improvement, even if f theamplifiers match the effective resistance, or

plate impedance, of the tubes to give maximum power output, but it maybe necessary to present lower impedances than this in order to obtain abroader fiat frequency response band. at the expense of lower powerconversion efiiciency. 'In other cases we may wish to operate theamplifiers with smaller power output and higher efiiciency by presentinga higher impedance load to the tubes. In any case, my invention makesitpossible to use a higher load resistance .and to obtain higherefi'iciency than can be obtained in ordinary amplifiers of the prior artwhen these i amplifiers are required to operate over an arbitrary wideband of frequencies.

It should be noted that the balancing out of reactive components occursboth in the inputand outputcircuits of the amplifiers A and B. This 1condition is especially desirable when the amplifier is used foramplifying currents extending over a wide range of frequencies.

Fig. 2 shows graphically the improvement :obtained in one particularembodiment constructed in accordance with the invention, over a singleamplifier. The curve labeled One standard amplifier shows the rate ofchange of current in a resistance load with variation of frequency whena single amplifier was used. The curve labeled Two broad band amplifiersshows the variation in current in the load when two amplifiers, eachhaving two tubes, were used in accordance with the present invention,one of said amplifiers being coupled to the load directly and the otherof said amplifiers being coupled through a quarter wave line, in themanner 'described and illustrated in connection with Fig. 1.

Of course, the two amplifiers were excited with inputs difiering by 90in phase,-as nearly as possible, by means of the quarter wave linesection between the input circuits, in the same manner described abovein connection with Fig. 1.

If we call the output to the load 100% when=the input frequency isadjusted to give maximum output, then we might set the limits of thefrequency band at the frequencies on either side of the frequency givingmaximum output at which the current in the load has dropped down to 70%of maximum. With this assumption, tests made on an arrangement like Fig.1 indicate that the band width of my amplifier was increased from 2.9megacycles to 5.03 megacycles by means of the invention. This increaseis equivalent to an improvement of about 1.7 3 to 1 in band width of theamplifier. If, .onthe other :hand, we set a more standard for frequencyresponse, than the ratio of improvement in band width will be greater.For example, if the allowable drop in -load --current is arbitrarilyallowed to reduce the load .current to other rpercentages :between 70%:and 100%, then according to tests made in the laboratory, the ratio ofincrease in band width due to the application 50f .the principlesof theinvention is given in the following table. T

j Ratio-of increase .inlband "Minimumslloweblc-percentagcloadcurrent 7width can It should be noted that the ratio of improvement is greaterwhen we set a more rigid band width requirement. {This condition, foundin practicacfollows :the theory which indicates that the more-severe thefrequency response require- .ment is made the greater will bethe ratioof improvement brought about by the invention. The reason for thisgreater improvement is that the quarter wavelength .line impedanceinverter gives more nearly perfect compensation .as the band widthisieduced .bysetting .a more severe requirement in establishing theallowable drop in load current .at .the limits .of the frequency band. r1

.At this time it may be advisable ,to .set .iorth very briefly the:manner inlwhich the -.,tes'ts.were .made upon which the curves .of 2are based. .The method :of making the tests involved apply- .ing acertain amount .of loadtooneof'the .amplifiers operatingindependentlyiof the other. Then the two amplifiers were joined togetherthrough the quarter Wavelengthli-nelsection and adjusted until the loadon each. amplifier was substantially ,,the sameas had been the load .onthe one amplifier a1one.- Qf course, the excitation .to each of I :thetwo amplifiers .wasmade substantially the same as that for one amplifieralone, which condition automatically assured substantially 90 phaserelation between :the high frequency inputs; For both the singleamplifier and for the system using the two amplifiers joined through thequarter wave line, the frequency of the input excitation was varied.over -a wide .band. The excitation source was one of .the wide bandtesting transmitters used at the laboratories for broad band antennadevelopments and tests. I It :should be noted that the load on theamplifier tubes was .not adjusted to obtain maximum ,possible band widthfrom each 'amplifienbut was adjusted simply to "obtain a fairly heavyload in .each case. The load resistance used consisted of two 'carbonlamps'which were applied with the shortest poslsiblerconnections to one:of the amplifier circuits. Fig. 3 illustrates how a chain of amplifierstages having quarter wave, quarter phase input .andmitput oircuitscanbewcascaded. Each stage includes two amplifiers .A and B, as in. Fig, 1,and is shown enclosed a box or electromagnetic shield With the output oftone-stage feeding the input of the succeeding stage. 7 An .arrang'ement as illustrated in Fig. 3 :is i1sef-ulin-broad 7 frequency bandamplifier systems and increases the band width and efficiency of thewhole system.

In the system of Fig. 3, it is preferable, though not essential, thatthe inductive input coupling shown in Fig. 1 be replaced by directconnections similar to those shown in the output circuit in order thatthere may be, no loss of band width due to leakage reactance'intheinductive coupling. 1 i

In constructing the transmitter systems of Figs. 1 and 3, it may happenthat there are required quarter wave line sections having characteristicimpedances of rather high values which are not easily reached withordinary types of transmission lines. Consequently, in these cases theline sections may be made out of coiled wire or coiled conductors sodimensioned as to give the desired impedances. I have found that a lineconstructed with coiled conductors may be given almost any desired valueof characteristic impedance over a wide range lying above the valueobtained with straight conductors of the same size. There may also beused artificial lines constructed with lumped coils and condensers.

In cases where it is desired to employ transmission lines of ordinaryimpedances obtainable without coiling, it is possible to lower theamplifier tube impedances as well as the load impedance by employing anumber of tubes in parallel for each tube of the amplifier stage.

In some cases the impedance of the line TL' required to give correctloading on the amplifiers may be too high or too low to be suitable fora long line to the antenna. In such cases we may transfer the impedanceto any other value, over a wide range, by means of a section oftransmission .line of continuously changing characteristic impedance asdescribed in my United States Patent No. 1,926,807, granted September12, 1933. We may accomplish a similar result not quite so well but moreeasily by employing two quarter wave line sections of differentimpedances in series.

What is claimed is: I

1. The method of operating a pair of electron discharge deviceamplifiers having substantially equal loading conditions at all inputand. output amplitudes which includes the step of exciting saidamplifiers with alternating current power at substantially a 90 phaserelation, and establishing substantially zero phase'relation between thetwo components of power derived from said amplifiers.

2. In an amplifier system having a pair of amplifier units, the methodof broadening the frequency response characteristic of said system whichcomprises adjusting said units to receive and deliver equal values ofpower at all amplitudes, and causing the reactive components in theinput and output circuits of one unit to react respectively upon thereactive components in the input and output circuits oi the other unitto greatly reduce the reactive component of current required from saidamplifier units.

3. The method of operating a pair of electron discharge deviceamplifiers having substantially equal input and output power atallamplitudes which includes the step of exciting said amplifiers withalternating current power at substantially a 90 phase relation,establishing substantially zero phase relation between the twocomponents of power derived from said amplifiers, and deriving usefulpower from the amplifier with lagging phase.

4. The method of operating a single stage of amplification comprising apair of similar and similarly adjusted amplifiers, which comprisesexciting said amplifiers from a single source of carrier frequencycurrent at a phase relation, whereby the reactivecomponents in the inputcircuits appearing on both sides of the carrier frequency are balancedout, producing a compensating phase shift of the amplified waves,whereby they combine in additive phase, and deriving useful power from apoint on that amplifier which has lagging phase. 7

5. An amplifier stage comprising a pair of similar and similarlyadjusted amplifiers -each having an input and an output circuit, autilization circuit coupled to said output circuits, and means forbalancing out the reactances appearing in said input and outputcircuits, said means comprising a section of transmission line whoselength is substantially one-quarter of a wavelength at the center of thefrequency band to be amplified connected between said input circuits,and a section of line having similar characteristics to said firstsection of line connected between said output circuits.

6. A broad frequency band amplifier stage comprising a pair of similarand similarly adjusted amplifiers, a section of transmission line whoselength is an odd multiple including unity of a quarter wave at thecenter of the frequency band to be amplified coupling together the inputcircuits of 'said amplifiers, and another section of line whose lengthis an odd multiple including unity of a quarter wave at the center ofthe frequency band to be amplified coupling together the output circuitsof said amplifiers.

'7. The method of operating a pair of electron discharge deviceamplifiers having substantially equal input and output power at allamplitudes which includes the step of exciting said amplifiers withalternating current power at substantially a 90 phase relation,establishing substantially zero phase relation between the twocomponents .of power derived from said amplifiers, and combining saidtwo components into a single output circuit.

8. A broad frequency band amplifier stage comprising two sets ofpush-pull connected amplifier tubes, each set comprising a pair ofelectron discharge devices having input and output electrodes, means forsimilarly biasing the input electrodes of said amplifier tubes wherebysaid two sets are substantially equally responsive for all valuesofexcitation voltage, a tuned circuit coupled to the input electrodes ofeach set for supplying out of phase potentials thereto, means forexciting said tuned circuits with equal values of alternating currentpower at all power values but at 90 phase relation, a path whoseelectrical length is an odd multiple including unity of a quarter waveat the center of the frequency band connecting the output electrodes ofone set to the output electrodes of the other set, and a utilizationcircuit coupled to the output electrodes of that set which has laggingphase.

9. A broad frequency band amplifier stage comprising two sets ofpush-pull connected amplifier tubes, each set comprising a pair ofelectron discharge devices having input and output electrodes, means forsimilarly biasing the input electrodes of said amplifier tubes wherebysaid two sets are substantially equally responsive for all values ofexcitation voltage, a. tuned circuit coupled to the input electrodes ofeach set for supplyingv out of phase potentials thereto,

means for exciting said tuned circuits with equal values of alternatingcurrent power at all power values but at 90 phase relation, a section oftransmission line whose electrical length is an odd multiple includingunity of a quarter wave at the center of the frequency band to beamplified connecting the output electrodes of one set to the outputelectrodes of the other set, and another section of transmission linecoupled to the output electrodes of that set which has lagging phase,said last section of line having a characteristic impedance which ishalf that of said first section of line.

10. A cascaded amplifier system comprising a first stage having two setsof push-pull connected amplifier tubes, each set comprising a pair ofelectron discharge devices having input and output electrodes, means forsimilarly biasing the input electrodes of said amplifier tubes wherebysaid two sets are substantially equally responsive for all values ofexcitation voltage, a tuned circuit coupled to the input electrodes ofeach set for supplying out of phase potentials thereto, means forexciting said tuned circuits with equal values of alternating currentpower at all power values but at 90 phase relation, a path whoseelectrical length is an odd multiple including unity of a quarter waveat the center of the frequency band connecting the output electrodes ofone set to the output electrodes of the other set, and a second similaramplifier stage coupled to the output electrodes of that set which haslagging phase.

11. Means for obtaining a broad band radio frequency amplifier systemcomprising two similar and similarly adjusted amplifiers supplied withradio frequency input through connections differing in electrical lengthand delivering power to a load through connectionsdiffering inelectrical length by an odd multiple including unity of a quarter of awave at the center of the frequency band to be amplified.

12. Means for obtaining a broad band radio frequency amplifier systemcomprising two similar and similarly adjusted amplifiers joined togetherat their inputs and outputs by sections of transmission line which areeach electrically onequarter or an odd multiple of one-quarter wavelong.

13. Means for obtaining a broad band radio frequency amplifier systemcomprising two similar and similarly adjusted amplifiers joined togetherat theininputs and outputs by impedance inverting networks or lines.

14. The method of operating a pair of electron discharge deviceamplifiers which includes the step of exciting said amplifiers withequal values of alternating current power at all amplitudes and atsubstantially a phase relation, and establishing substantially zerophase relation between the two components of power derived from saidamplifiers.

15. In an amplifier stage having two similar and similarly adjustedamplifier units interconnected with impedance inverting circuits, themethod of broadening the frequency response characteristic of saidamplifier stage which comprises balancing out the reactive components inthe input and output circuits of said amplifier stage.

16. A broad frequency band amplifier stage comprising two sets ofpush-pull connected amplifier tubes, each set comprising a pair ofelectron discharge devices having input and output electrodes, means forsimilarly biasing the input electrodes of said amplifier tubes wherebysaid two sets are substantially equally responsive for all values ofexcitation voltage, a tuned circuit coupled to the input electrodes ofeach set for supplying out of phase potentials thereto, means forexciting said tuned circuits with equal values of alternating currentpower at all power values but at 90 phase relation, saidlast meansincluding a line whose electrical length is an odd multiple includingunity of a quarter wave at the center of the frequency band, a pathwhose electrical length is an odd multiple including unity of a quarterwave at the center of the frequency band connecting the outputelectrodes of one set to the output electrodes of the other set, and a'utilization circuit coupled to the output electrodes of that set whichhas lagging phase.

17. In an amplifier system having a pair of amplifier units, the methodof operation which comprises adjusting said units to receive eq valuesof power at all amplitudes at a desired phase relation and to amplify ua1' va1ues of' power at all amplitudes W desired phase

